The present invention relates to a method for sealing the terminal post of a storage battery and a post seal assembly for use therein. The invention is particularly adaptable to sealing the interface between a lead terminal post and the terminal opening in a plastic cover for a lead-acid battery.
It is well known in the art of secondary lead-acid batteries that one of the most difficult and vexatious problems is the construction and maintenance of a liquid-tight seal between the battery terminal post and the container cover through which the post extends from the interior of the battery. One of the typical seals in common use today comprises a lead bushing which is insert-molded into the container cover and into and through which the cylindrical lead terminal post extends as the cover is placed on and sealed to the container. The interface betwenn the ID of the lead bushing and the OD of the terminal post is subsequently heat-fused to provide an integral structure and tight fluid seal.
However, as is well-known in the art, the terminal post, which is integrally attached at its lower end to battery cell elements in the interior of the container, is subjected to a substantial axial force as a result of the expansion of the plates comprising the elements during cycling of the battery. Because of the rigid attachment of the terminal post to the lead bushing and, in turn, the lead bushing to the container cover, the axial force on the terminal post often results in failure of one of the rigid interfaces or cracking of the cover itself. In either event, the insidious migration of acid electrolyte along surface imperfections in the various components of the seal is obviously worsened in the event of failure of the sealing surfaces or cracking of the cover. The migrating acid electrolyte will eventually find its way past the seal and result in the condition known as "post-leakage" which leads to corrosive damage of the terminal connection and increased electrical resistance.
In recognition of the foregoing problems, many attempts have been made to provide a fluid-tight seal between the terminal post and battery cover which allows vertical movement of the post. In a general sense, these prior art seals all employ a flexible rubber or plastic sealing member between the terminal post and the lead bushing or between the bushing and an integral plastic sleeve molded as part of the cover and defining the terminal post opening therein. Attempts have also been made to seal directly the interface between the terminal post and the cover by molding, pressing, or otherwis inserting a flexible sealing material therebetween.
One prior art seal assembly, disclosed in U.S. Pat. No. 4,522,899, comprises a composite annular plastic ring which is molded in two steps around the terminal post. The first molded component of the composite is a soft thermoplastic rubber which is subsequently enclosed by molding a harder thermoplastic material, such as polypropylene, around it and the terminal post. Shrinkage of the second molded thermoplastic material around the softer first material compresses the latter to enhance the seal with the terminal post and between the two plastics forming the composite. The container cover is placed over the terminal post which extends therethrough and the underside of the cover is welded to the upper surface of the seal assembly as the cover is heat-sealed or ultrasonically welded to the battery container. The effectiveness of the foregoing seal assembly relies solely on the compression provided by the injection molding process and subsequent shrinkage of the outer harder thermoplastic component during hardening and cooling.
U.S. Pat. No. 4,410,610 discloses a seal assembly in which the lead bushing is encased in a rubber sealing material along its entire outer diameter, lower end and most of its inner diameter. The seal is inserted around a terminal post extending through the cover and inserted in a cup-shaped depression in the cover surrounding the post. The exposed upper portion of the ID of the lead bushing is subsequently welded or burned to the terminal post such that the composite lead and rubber bushing is rigidly fixed to the terminal post. Vertical movement of the terminal post is accommodated by movement of the rubber OD portion of the seal within the cup-like depression in the cover. Although this seal assembly appears to provide an effective barrier against acid migration directly along the terminal post, the flexible and movable interface between the assembly and the cover, because it is not a highly compressed seal, would appear to provide a ready alternate path for acid migration to the outside of the cover.
U.S. Pat. No. 4,495,260 describes a terminal post seal assembly somewhat like that disclosed in U.S. Pat. No. 4,410,610 above. A lead bushing is molded into a larger diameter plastic bushing and the composite assembly is inserted over the terminal post and into a cup-like depression in the container cover. The lead bushing is fused to the terminal post and the interface between the outer plastic bushing and the cylindrical surface of the cup-like depression in the cover is constructed for vertical movement and sealed with an O-ring or rings. A terminal post seal utilizing O-rings as the primary sealing elements is also disclosed in U.S. Pat. No. 4,164,609. In this construction, however, O-rings provide the seal between both the terminal post and the surrounding bushing-like retainer assembly and between the retainer assembly and the portion of the cover housing the assembly. O-rings are known to distort and provide non-uniform sealing when compressed. In addition, O-rings have an inherently small sealing surface area which is generally considered to be unsatisfactory for the difficult sealing problem presented by the terminal post of a lead-acid battery.
Thus, whether utilizing constructions in which the terminal post is rigidly fixed to the cover or where a seal assembly is used which permits relative movement between the post and the cover, acid migration and its resultant adverse effects remain problems. The prior art dynamic seals which permit vertical movement of the terminal post do so at a substantial sacrifice in sealing capability. A dynamic seal of this type, to be effective, is believed to require a significant level of compression against the surfaces to be sealed which compression must be controlled and maintained during the operational life oi the battery. It is also desirable that the seal assembly be of relatively simple construction and easy to install.